WMD is a lightweight HTML editor which is ideal for blog comments, forum posts & basic content management.
It can be implemented into any textarea with one line of code & with an extra line, live preview functionality can be implemented.

This is not a WYSIWYG editor. Rather, it is a WYSIWYM Markdown editor (what you see Is what you mean).
WMD produces clean semantic HTML, leaving presentation details like fonts and colors up to style sheets.
Markdown: a simple markup language that’s as easy to read and write as plain-text email.
Requirements: No Requirements
Compatibility: All Major Browsers
Xinha is a powerful & open source HTML (WYSIWYG) editor.
It is a community-built script presenting various options for max. customization.

It has lots of plugins that can be activated like:
- Character counter
- Find & replace
- Image manager
- HtmlTidy
- Spell checker & more.
It is possible to configure which features/menus to be displayed, whether coding inside will be permitted or not & almost anything that may be needed.
Requirements: No Requirements
Compatibility: All Major Browsers
Corpocrat is presenting 2 simple & handy editors:

They both convert a textarea into an editor & they are lightweight (<5kb).
Links & images are inserted with an alert window rather than a HTML popup.
And, they are free to be used in both personal & commercial projects.
Requirements: No Requirements
Compatibility: All Major Browsers